Bob Dolph Answers Questions on Mounting Switches and DC-Powered Sirens

Q: Bob La Londe (Yuma, AZ) recently asked, “For years I have realized I don’t necessarily mount switches right. Panel mount toggle and rocker switches have a flat spot on one side to keep them from spinning. How do you make a hole to take advantage of that?”

A: Bob, you need to use a chassis hole punch that is specially designed for “D-shaped” holes. Many distributors carry these punches or you can get additional information from Greenlee ( It is a screw-driven punch that you use a wrench to compress and cut a very clean D-shaped hole. I have even used them in the past to cut key-switch holes on an automotive chassis.
Another lower cost solution would be to mount the switch in a standard round hole using a star-washer under the switch nut. The washer would bite into the chassis and keep the switch from spinning. Make sure on exterior applications to coat punched bare metal so as to protect them from rusting.

Q: I am having trouble with a DC powered siren. The voltage at the siren is below the specified range when the alarm panel is in alarm. I don’t understand what could be the problem since the siren is one that is specified by the panel manufacturer. What could I be doing wrong?

A: DC sirens can draw a considerable amount of current. There is a good chance that you either have a highly resistive bad wire connection or the gauge of wire you are using is too small in size for the distance of the siren cable run.
Don’t forget Ohm’s law — E=IR. Any time current is pulled through a wire, there will be a voltage drop due to the resistance of the wire. Look at the formula — if resistance (R) is constant, then as the current increases the voltage will.
Remember that all voltage drops (both the wire and siren load) must equal the panel or source voltage. Too much of a voltage drop in the wire will steal needed minimal voltage for the siren to operate. Either increase the size of the wire (decrease the gauge number) or find a siren that can operate at a lower voltage or draw less current. Sometimes you can double up spare wires to increase the overall size of the cable.
You may also want to look at using a speaker siren, where the siren driver is in the panel.

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